Of various printing methods, a lithographic printing method has enjoyed general use because of the ease in platemaking and has now furnished the primary printing means. The lithographic printing technique, being based on the immiscibility of oil and water, uses a printing plate selectively holding an oily material, i.e., ink, on the image area and a fountain solution on the non-image area. On contact with a printing substrate directly or indirectly via an intermediate called a blanket, the ink on the image area is transferred to the printing substrate, such as paper, to achieve printing.
The lithographic printing method primarily consists in preparation of a presensitized plate (PS plate) which comprises an aluminum plate as a support coated with a diazo photosensitive layer. In the preparation of a PS plate the surface of an aluminum plate is subjected to graining, anodizing, and other various processings to establish ink-receptivity for the image area and ink-repellency for the non-image area, to improve impression capacity and to improve precision. Therefore, lithographic printing has acquired the character of high impression capacity and of high precision as well as the ease of platemaking.
However, with the spread of printed matter, there has still been a demand in order to further ensure the ease of lithographic printing, and a number of printing methods have been proposed, seeking ease of platemaking.
Amongst the proposals typical is a printing method using a printing plate prepared by silver salt diffusion transfer as disclosed in U.S. Pat. No. 3,511,656 and JP-A-7-56351 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Such a lithographic printing plate material includes "Copyrapid" available from Agfa-Gevaert AG. This method has been put to practical use as an easy printing method because a transfer image can be formed through one step, and the resulting transfer image, having ink-receptivity, can serve for printing as such. However easy it is, the method requires a step of diffusion transfer development with an alkali developer. Hence, an easier printing method that does not require a developing step with a developer has been anxiously awaited.
In the light of the above-mentioned situation, a simplified method of platemaking in which a developing processing with an alkali developer after imagewise exposure is omitted has been developed. In the field of this type of printing plates, which are called non-processed plates, the principal means that have been proposed to data for image formation are based on (1) imagewise exposure of a recording surface to cause thermal destruction of the irradiated area, (2) imagewise exposure of a recording surface to make the irradiated area ink-receptive through heat mode hardening, (3) imagewise exposure of a recording surface to make the irradiated area ink-receptive through light mode hardening, (4) change of surface properties through photolysis of a diazo compound, and (5) heat mode fusion heat transfer of an image area.
While the techniques listed above give manipulations for avoiding the necessity for a developer, they involve at least one of the disadvantages that (1) the difference between a lipophilic area and a hydrophilic area is insufficient and that (2) the image area is easily scratched due to insufficient mechanical strength. The disadvantage (1) causes inferior print quality and low resolution and makes it difficult to obtain a printing plate with excellent image sharpness. The disadvantage (2) necessitates formation of a protective film, which ruins the ease of platemaking, or results in insufficient durability withstanding long-term printing. In brief, mere omission of an alkali development processing from platemaking operations has failed to provide a practically useful printing method. The keen demand for a method for providing with ease a printing plate satisfying various requirements has not been fulfilled yet.
JP-A-9-169098 discloses a platemaking method taking advantage of zirconia ceramic's becoming hydrophilic on irradiation with light, which is one of non-processing platemaking techniques. However, zirconia has insufficient photosensitivity, and change from hydrophobic to hydrophilic properties by light is insufficient for creating a clear distinction between image areas and non-image areas.
In addition to a simplified platemaking method requiring no developer, a means enabling regeneration of a used printing plate for reuse would be of great advantage for reduction of cost and waste. Reuse of a printing plate relies for utility on the simpleness and ease of the operation for regeneration. To regenerate a used printing plate without a complicated operation is a technically difficult problem that has received little study. The only teaching is found in JP-A-9-169098 supra, which deals with a special material for a printing plate precursor, zirconia ceramics.